Determine if global tables should be upgraded.

This function performs a series of checks to ensure the environment allows
for the safe upgrading of global WordPress database tables. It is necessary
because global tables will commonly grow to millions of rows on large
installations, and the ability to control their upgrade routines can be
critical to the operation of large networks.

In a future iteration, this function may use `wp_is_large_network()` to more-
intelligently prevent global table upgrades. Until then, we make sure
WordPress is on the main site of the main network, to avoid running queries
more than once in multi-site or multi-network environments.

function wp_should_upgrade_global_tables() {

	// Return false early if explicitly not upgrading.
	if ( defined( 'DO_NOT_UPGRADE_GLOBAL_TABLES' ) ) {
		return false;
	}

	// Assume global tables should be upgraded.
	$should_upgrade = true;

	// Set to false if not on main network (does not matter if not multi-network).
	if ( ! is_main_network() ) {
		$should_upgrade = false;
	}

	// Set to false if not on main site of current network (does not matter if not multi-site).
	if ( ! is_main_site() ) {
		$should_upgrade = false;
	}

	/**
	 * Filters if upgrade routines should be run on global tables.
	 *
	 * @since 4.3.0
	 *
	 * @param bool $should_upgrade Whether to run the upgrade routines on global tables.
	 */
	return apply_filters( 'wp_should_upgrade_global_tables', $should_upgrade );
}